Thermostats for heating and/or air-cooling systems generally include means for storing a desire setpoint temperature, means for measuring the actual temperature within the building, and means for switching the heating or cooling unit on or off as a function of differences between the setpoint temperature and the actual temperature. Advances in microelectronics have led to development of programmable electronic thermostats which allow the user to program a schedule of temperature setpoints corresponding to selected times in a repetitive cycle. A principal advantage of programmable thermostats is that the user may program his thermostat to automatically set back the temperature of his building during selected periods of the day in order to conserve energy.
However, much of the potentially significant savings in energy costs may be undermined by the inefficient operation of an air-cooling system controlled by the thermostat. For example, the thermostat may be programmed to maintain the building at a temperature of 72.degree. during periods of the day when persons are likely to be home, and maintain the temperature at 82.degree. during periods when the building is empty. When the thermostat energizes the air-cooling unit to lower the room temperature to 72.degree. from the setback point of 82.degree. the unit may operate for an extended period of time in order to get back to the programmed setpoint. Extended continuous operation of the air-cooling unit is likely to result in the accumulation of ice on the evaporator and, thus, a significant loss in operating efficiency. This loss in operating efficiency requires the air cooling unit to operate for a still longer period of time in order to achieve the programmed setpoint, thus, undermining the energy savings achieved by setting back the temperature.
It is therefore an object of the present invention to provide an electronic thermostat including means for controlling the operation of an air-cooling unit to prevent the accumulation of ice on the cooling element of the unit thereby increasing the operating efficiency of the system.
It is another object of the present invention to provide an electronic thermostat having an automatic de-icing cycle which deenergizes the operation of the cooling element in the air-cooling unit for a first preselected period of time whenever the unit has operated continuously for a second preselected unit of time.
It is another object of the present invention to provide an electronic de-icing thermostat including means for adaptively modifying the length of either or both of the first and second preselected time periods in response to sensed temperature conditions in order to maximize the operating efficiency of the system.